1. Field of the Invention
The present invention relates to a semiconductor memory device having a gate structure with a floating gate and a control gate stacked, and also relates to a method of manufacturing the same.
2. Description of the Related Art
Semiconductor memory devices known in the art include electrically programmable non-volatile semiconductor memory devices (EEPROMs). Among those, attention has been given to an NAND-type EEPROM, because it can be highly integrated, which includes an NAND cell having a plurality of serially connected memory cells each serving as a unit for one bit storage. The NAND-type has been widely utilized, for example, in memory cards for storing image data from digital still cameras.
A memory cell in the NAND-type EEPROM has a MOSFET structure with a floating gate (charge accumulation layer) and a control gate (CG) stacked via an insulator on a semiconductor substrate serving as a channel region. A memory cell unit comprises a circuit with a plurality of memory cells serially connected such that adjacent ones share an impurity region (source/drain), and selection gate transistors connected to both ends of the serial circuit, respectively. One of the selection gate transistors has a drain connected to a bit line extending in parallel with the direction of memory cells arrayed, and the other of the selection gate transistors has a source connected to a source line. The control gates of the memory cells form word lines extending orthogonal to the bit line, and the gates of the selection gate transistors form selection gates extending orthogonal to the bit line. The control gates of the memory cells and so forth contained in the word lines are usually controlled to exhibit lower resistances through formation of silicide (see JP 2004-63789A, paragraph 0031, FIG. 25, for example).
In such the semiconductor memory devices of the prior art, fine pattering of word lines in accordance with fine-pattered memory cells in recent years causes a problem associated with word lines having increased resistances. In particular, if the word lines are silicided, an increase in resistance variation due to unevenness among metal silicide films, and deterioration due to agglomeration may cause a problem. This problem is regarded as a result of a number of metal silicide films produced in an extremely small area.